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The results from the small Australian study were only modest. But they back up observational studies showing potential mental benefits from physical activity.

The effects of exercise were at least as good, if not better, than those seen with drugs approved to aid mental function in Alzheimer's disease, according to experts not involved in the study.

Still, the study authors cautioned that the results don't prove that exercise will produce meaningful improvement in brain function or memory. They also said the results should not be used to imply that exercise reduces the risk of dementia or Alzheimer's — that can't be determined from this type of study.

The authors said it's not clear how exercise might affect brain function; one theory is that it improves blood flow to the brain. Their study did not involve brain imaging that would have shown any changes in blood flow associated with exercise.

Results appear in Wednesday's Journal of the American Medical Association.

Study participants included 85 Australian adults aged 50 and older assigned to do at least two hours of weekly physical activity, mainly brisk walking, for six months. They recorded their exercise in diaries and also got phone calls and newsletters encouraging them to stick with it.

They were urged to remain active even after the initial six months, and were compared with a control group of 85 people who were not asked to exercise.

The exercise group engaged in about 20 minutes more activity a day than the control group.

After six months, the exercise group performed 1.3 points better on a 70-point scale of brain function than the non-exercise group. The effects remained at 18 months, though the difference by then, about 0.7 points, was minimal.

"To our knowledge, this is the first randomized clinical trial being published" on exercise and brain function in older adults with problems, said the lead author Dr. Nicola Lautenschlager of the University of Melbourne.

"It's an important piece in that it's the first intervention in people with memory complaints that's showing some potential benefit," said Dr. Raj Shah, director of the memory clinic at Rush University Medical Center. He was not involved in the study.

Shah said larger studies are needed before physical exercise would be prescribed to treat memory problems in older adults.

Dr. Ron Petersen of the Alzheimer's Association agreed that the study should not be "overhyped," but said he will use the results in discussing potential benefits of exercise with patients.